Integration of size distributions and size-resolved hygroscopicity measured during the Houston Supersite for compositional categorization of the aerosol

Measurements of aerosol size distributions and hygroscopicity were combined to infer the size-resolved composition of the ambient aerosol during the Houston Supersite campaign from June through October, 2001. The 3000+ distributions used in this analysis were measured at the Aldine site north and typically downwind of the greater Houston metropolitan area. Size distributions spanning the diameter range from 0.025 to 0.700 μm, and hygroscopic behavior at eight logarithmically spaced dry diameters from 0.025 to 0.344 μm were analyzed. At smaller dry sizes, the aerosol hygroscopic growth factor distributions were typically monomodal and peaked at or near a growth factor of 1.0, indicating predominantly non-hygroscopic aerosols. Particles larger than 0.100 μm exhibited bimodal growth patterns, with increasing importance of the hygroscopic mode with increasing dry particle size. Hygroscopic growth distributions were used to partition the aerosol into pure insoluble, mixed insoluble, mixed soluble, and pure soluble categories. This categorization scheme was used to analyze in detail four multi-day episodes. During two of these episodes new particle formation events were observed daily. The recently formed particles were only sparingly hygroscopic, suggesting they were composed primarily of insoluble material. To contrast these periods during which pronounced diurnal variability was observed, a third period was chosen because the aerosol concentration and composition varied little. The fourth period analyzed spanned the passage of a cold front, which had a pronounced influence on the aerosol distributions.